I have lived this game and can offer you the following.
You may be able to use the MAC sound input for the voiceover tool. You can set the VO tool to use a different input to the log and capture tool in FCP. This works on one of our Dual G5’s. The other machine is nominally identical and it doesn’t work.
If you use the Decklink input as the input for the VO tool you will need to turn off the external monitor when you do recordings (under the view menu, external video – set to off). The trick is to get the external monitor to show black. If it shows anything other than black (i.e. no signal, not black video) say the desktop, you can’t record. Apparently this is the way Apple have set up the tool to work. This means you can’t do ADR or anything else that requires you to have an external monitor working as you record.
You can also record audio only using the log and capture tool, but you have to supply a valid video signal, even if you aren’t recording it.
Hope this helps.
PS. I would be interested to hear if you can get the MAC input working. I still can’t figure out why it works on one of our machines and not the other.
Trevor Kinsey,
Technical Producer,
CVP Film and Television,
Melbourne,
Australia